Are you still bearing healthy fruit? Turns out that it's not just women who have a biological clock—your sperm may be going to seed a lot faster than you think.

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While you've never been against the idea of a serious relationship, you are in no particular rush to become a schlub. The attendant trappings of new fatherhood—the preschool viewings, the sleepless nights, the humiliation of carrying a diaper bag—aren't exactly calling out to you the way, say, another night slinging Pisco sours would. The ever-intensifying din of the proverbial biological clock? That's for the opposite sex to worry about—you know, like periods, frizz, and whether Mr. Big will dump Carrie in the Sex and the City sequel. As far as you know, your little swim team of DNA carriers will be competing at Olympic level into Letterman age. So what's the rush?

"I always thought my biological clock was the 36 hours I had left after I took my Cialis pill," says Zack, a 30-year-old producer in Los Angeles. "That's the only clock I've ever felt ticking." Turns out, Zack might want to consider the unsung glories of fatherhood.

According to a study released last March in the Public Library of Science Medicine, children born to fathers who were 20 scored an average of 2 points higher on an IQ test than children born to 50-year-old fathers. And that's not all. Recent studies from Israel, California, and Sweden have connected "late paternal age" with any number of serious medical conditions: The longer you wait, the more likely it is that your kid will be affected by schizophrenia, dwarfism, bipolar disorder, autism, Marfan syndrome, certain childhood cancers, or even, later in life, Alzheimer's. In some cases, the risk factors skyrocket. A 2005 study conducted by the University of California, Los Angeles, found a fourfold rise in Down syndrome among babies born to men 50 and older. Worse still, those risk factors aren't limited to your tweed-sporting years: Statistically, "late paternal age" starts at 30, as in Zack's age. A 2006 study conducted by Mount Sinai School of Medicine found that fathers in their thirties have children with about 1.5 times the risk of developing autism compared with fathers in their teens and twenties. That factor jumps to five times for dads in their forties. The cherry on the cake? The American Society for Reproductive Medicine recommends that sperm banks do not accept specimens from men over 40.

"The biological clock for men and women is really the same," says Dr. Dolores Malaspina of Bellevue Hospital Center in New York City and New York University, who conducted one of the first studies. "It's just that men can keep having babies."

The biology behind this isn't hard to grasp: Starting in puberty, spermatogonia, the master copies for sperm production, replicate themselves every couple of weeks. After 300 to 500 copies—somewhere in your thirties—a meaningful number of small copy errors, or point mutations, start to emerge, which accumulate over time.

Yet, despite the alarming new science, most men greet parenthood with a sense of urgency that's more in line with Zack's than Angelina Jolie's. The reason is simple: While women are inculcated with the risks of late-age motherhood in sixth-grade sex ed, men remain blissfully ignorant. Since the recent studies have been published, the bad news still doesn't seem to be making it to the doctor's office. Scott, a 32-year-old schoolteacher from Babylon, New York, decided to start a family when he was Zack's age, strictly because he wanted to raise his child while he was young. "For me the doctors were like, 'Hey, this is going to be good. You're still active,'" Scott says. "Nobody ever told me about the medical risks of being an older dad."